Methods of evaporating metal onto a semiconductor wafer in a test wafer holder

ABSTRACT

Apparatus and methods for evaporating metal onto semiconductor wafers are disclosed. One such apparatus can include an evaporation chamber that includes a wafer holder, such as a dome, and a test wafer holder that is separate and spaced apart from the wafer holder. In certain implementations, the test wafer can be coupled to a cross beam supporting at least one shaper. A metal can be evaporated onto production wafers positioned in the wafer holder while metal is evaporated on a test wafer positioned in a test wafer holder. In some instances, the production wafers can be GaAs wafers. The test wafer can be used to make a quality assessment about the production wafers.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/898,632, filed Oct. 5, 2010, entitled “APPARATUS AND METHODS FOREVAPORATION INCLUDING TEST WAFER HOLDER,” the disclosure of which ishereby incorporated by reference in its entirety.

BACKGROUND

1. Field

The disclosed technology relates to systems for processing semiconductorsubstrates and, in particular, to systems for evaporating metal ontosemiconductor substrates.

2. Description of the Related Art

Processing of semiconductor substrates, such as GaAs wafers, may includedepositing a relatively thin film of metal onto the substrates byevaporation. Evaporation may be accomplished by heating a metal sourceand having the resulting metal vapor travel through a vacuum onto thesubstrate. The metal vapor may then condense on the substrate to producea thin film of metal. The thin film may be used for a variety ofpurposes, such as serving as a contact for a collector, a base, and/oran emitter of a bipolar transistor.

While manufacturing wafers that will be used to make production parts, atest wafer may also be processed in order to make a quality assessmentassociated with the production parts and/or to ensure that processes,such as evaporation, were carried out within acceptable bounds. Smallchanges in environment may noticeably impact performance and/orreliability of end products produced from semiconductor wafers. Thus,the test wafer can be positioned in tools, such as an evaporator, sothat the test wafer can be exposed to substantially the same processingenvironment as production wafers. As wafer diameter increases, fewerwafers can be positioned within the same amount of space. This leavesless space for test wafers, which may take away valuable space fromproduction wafers.

While evaporation onto substrates takes place, sensors, such as crystalmonitor sensors, may monitor conditions of evaporation. For example, acrystal monitoring system can detect a deposition rate and/or thicknessof metal deposited onto the substrate. For accurate measurements, thecrystal monitor sensor may be positioned near the production wafers sothat the crystal monitor is exposed to substantially the sameenvironment as the production wafers within an evaporation chamber.Information obtained using the crystal monitor sensor may be used tostop evaporation and prevent additional metal from being deposited ontothe substrates.

SUMMARY OF CERTAIN INVENTIVE ASPECTS

The apparatus and methods described in the claims each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope of this invention, some prominentfeatures will now be briefly discussed.

One aspect of the disclosure is a semiconductor processing apparatusthat includes a wafer holder, a test wafer holder, and a heat source.The wafer holder can be configured to hold a plurality of wafers, andthe wafer holder can be positioned within a chamber. The test waferholder, which can be configured to hold a test wafer, can be positionedwithin the chamber separate and spaced apart from the wafer holder. Theheat source can be configured to heat a metal source so as to evaporatemetal within the chamber onto a plurality of semiconductor waferspositioned in the wafer holder and a test wafer positioned in the testwafer holder.

According to certain implementations, the wafer holder is configured tohold at least one GaAs wafer having a diameter of at least about 150 mm.In some implementations, the test wafer holder is configured to hold awafer having a thickness of no greater than about 800 μm.

In accordance with various implementations, the apparatus can alsoinclude a shaper supported by a cross beam, wherein the test waferholder is coupled to the cross beam. In some of these implementations,the test wafer holder is positioned between the material beingevaporated and the shaper. The test wafer holder can include a top and abottom portion spaced by a plurality of posts, the bottom porting havinga hole surrounded by a ridge, the ridge configured to hold the testwafer in accordance with certain implementations.

According to a number of implementations, the wafer holder may include adome. In some instances the dome is configured to rotate.

According to several implementations, the metal source includes at leastone of gold, gold germanium alloy, platinum, titanium, nickel, andcopper. In various implementations, the apparatus is configured toprocess at least one GaAs wafer including at least a portion of a HBTdevice. In some of these implementations, apparatus is configured toprocess at least one GaAs wafer including at least a portion of a BiFETdevice. The apparatus can also be configured to process at least onesemiconductor wafer including at least a portion of at least one of avaractor, a gain block, and a pseudomorphic high electron mobilitytransistor (pHEMT) device.

According to some implementations, the apparatus can also include acrystal monitor configured to detect metal vapor associated with a metalsource prior to a portion of metal from the metal source being depositedon at least one wafer in the chamber; and a shutter configured to remainin a closed position in response to the crystal monitor detecting acondition, so as to prevent metal deposition on one or more wafers inthe chamber.

Another aspect of the disclosure is a method of processing asemiconductor wafer. The method includes providing a chamber including:a dome configured to hold a plurality of wafers including thesemiconductor wafer, and a test wafer holder separate from the dome;heating a metal source so as to evaporate metal from the metal sourcewithin the chamber; and evaporating metal onto a semiconductor testwafer positioned in the test wafer holder, while evaporating metal ontothe semiconductor wafer positioned in the dome.

In some instances, the semiconductor wafer is a GaAs wafer. A diameterof the semiconductor wafer is at least about 150 mm in certaininstances. And in various instances at least a portion of the metalevaporated onto the GaAs wafer is configured to function as a contact ona HBT device for at least one of a collector, a base, and an emitter. Ina number of instances, the test wafer is a Si wafer.

In accordance with some implementations, the test wafer holder isattached to a cross beam supporting a shaper, and wherein the shaper hasa footprint about at least as large as a footprint of test wafer holderin the path metal vapor travels between the metal source and thesemiconductor wafer positioned in the dome.

According to some implementations, the method also includes detecting acondition related to metal evaporation using a crystal monitor; andkeeping a shutter in a closed position in response to detecting thecondition, so as to prevent metal deposition of the metal source ontothe GaAs wafer and the test wafer in the chamber.

One more aspect of the disclosure is a method of processing a GaAswafer. The method includes providing an evaporation chamber including adome having slots configured to hold a plurality of wafers having adiameter of at least about 150 mm, and a test wafer holder configured tohold at least one test wafer, the test wafer holder spaced apart fromthe dome; positioning the GaAs wafer in a slot of the dome; applying anenergy source to a metal source, so as to evaporate metal from the metalsource within the evaporation chamber; and depositing a thin film ofmetal from the metal source onto the GaAs wafer, the GaAs including atleast a portion of at least one of a BiFET device, a varactor, a gainblock, and a pseudomorphic high electron mobility transistor (pHEMT)device.

Another aspect of the disclosure is a semiconductor processingapparatus. The apparatus includes an energy source, a wafer holder, acrystal monitor sensor, and a shutter. The energy source is configuredto heat a metal source so as to evaporate at least a portion of themetal source. The wafer holder is configured to hold at least onesemiconductor wafer, the wafer holder positioned in an evaporationchamber. The crystal monitor sensor is configured to detect metal vaporassociated with the metal source prior to a portion of metal from themetal source being deposited on the at least one semiconductor wafer. Inaddition, the shutter configured to remain in a closed position, inresponse to the crystal monitor sensor detecting a condition, so as toprevent metal deposition on the at least one semiconductor wafer.

In a number of implementations, the semiconductor wafer is a GaAs wafer.In some of these implementations, the wafer holder is configured to holdthe GaAs wafer having a thickness of no greater than about 800 μm.

According to various implementations, the apparatus also includesanother crystal monitor sensor configured to monitor conditions withinthe evaporation chamber during metal deposition onto the at least onesemiconductor wafer, wherein the shutter is configured to close inresponse to the another crystal monitor sensor detecting that a thinfilm of a predetermined thickness has been deposited onto the at leastone semiconductor wafer. In some implementations, the shutter isconfigured to remain closed only if the crystal monitor sensor detectsthe condition. In some implementations, the wafer holder is configuredto hold a wafer having a diameter of at least about 150 mm.

In accordance with a number of implementations, the condition isindicative of a composition of the metal source. According to a numberof implementations, the shutter is configured to open only if thecrystal monitor detects a predetermined metal source. In variousimplementations, the crystal monitor sensor is configured to detect aplurality of metal sources based at least in part on an input associatedwith each of the plurality of metal sources. In certain implementations,the condition is indicative of a position of the metal source within acrucible.

Yet another aspect of the disclosure is a method of processing a GaAswafer. The method can include heating a metal source within anevaporation chamber. The method can also include detecting an undesiredcondition for metal evaporation using a crystal monitor sensor. Inaddition, the method can include keeping a shutter in a closed positionin response to detecting the undesired condition, so as to prevent atleast a portion of the metal source to be evaporated on the GaAs waferin the evaporation chamber. A thin film of metal from the metal sourcecan be evaporated onto the GaAs wafer according to the method.

In some implementations, the undesired condition is associated with anevaporation rate of the metal source. According to a number ofimplementations, the undesired condition is associated with an impuritylevel of the metal source. In accordance with various implementations,the undesired condition is associated with a particular metal sourceused to form a contact on at least one of a collector, a base, and anemitter on a heterojunction bipolar transistor (HBT) device on the GaAswafer.

The shutter is configured to remain in a closed position only if thecrystal monitor sensor does not detect the particular metal sourceaccording to some instances. The undesired condition is associated witha position of the metal source in a number of instances.

In some implementations, the method also includes detecting a secondcondition associated with depositing a suitable thin film on the GaAswafer with another crystal monitor sensor; and closing the shutter inresponse to detecting the second condition.

The GaAs wafer has a diameter of at least about 150 mm in variousinstances. The GaAs wafer includes at least a portion of a bipolar fieldeffect transistor (BiFET) device according to some implementations. Inaddition, the shutter is configured to substantially prevent evaporationof the metal source onto the GaAs wafer when the shutter is in a closedposition in a number of implementations.

For purposes of summarizing the disclosure, certain aspects, advantagesand novel features of the inventions have been described herein. It isto be understood that not necessarily all such advantages may beachieved in accordance with any particular embodiment of the invention.Thus, the invention may be embodied or carried out in a manner thatachieves or optimizes one advantage or group of advantages as taughtherein without necessarily achieving other advantages as may be taughtor suggested herein.

The present disclosure relates to U.S. patent application Ser. No.______ [Attorney Docket SKYWRKS.102A], titled “APPARATUS AND METHODS FORDETECTING EVAPORATION CONDITIONS,” filed on even date herewith and ishereby incorporated by reference herein in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a side view of a wafer.

FIG. 2 depicts an enlarged view of a portion of the wafer of FIG. 1.

FIG. 3 illustrates an evaporator for depositing metal onto a pluralityof substrates, such as wafers, according to one embodiment.

FIG. 4 illustrates an evaporator including a test wafer holder separatefrom a wafer holder according to another embodiment.

FIGS. 5A and 5B show an example test wafer holder design according toone embodiment.

FIG. 6 is a flowchart of a method of processing one or more GaAs wafersusing an evaporator, such as the evaporators illustrated in FIGS. 3 and4, according to an embodiment.

FIGS. 7A and 7B illustrate an evaporator with two crystal monitorsensors according to an embodiment.

FIG. 8A shows an example shutter system according to an embodiment.

FIG. 8B shows an example crystal monitor sensor according to anembodiment.

FIG. 9 is a flowchart of a method of depositing method onto a waferthrough evaporation according to an embodiment.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

The headings provided herein, if any, are for convenience only and donot necessarily affect the scope or meaning of the claimed invention.

Provided herein are various methodologies and devices for processingwafers such as semiconductor wafers. In the description herein, variousexamples are described in the context of GaAs substrate wafers. It willbe understood, however, that some or all of the features of the presentdisclosure can be implemented in processing of other types ofsemiconductor wafers. Further, some of the features can also be appliedto situations involving non-semiconductor wafers.

In the description herein, various examples are described in the contextof front-side processing of wafers. It will be understood, however, thatsome or all of the features of the present disclosure can be implementedin back-side processing of wafers.

FIG. 1 depicts a side view of a wafer 30 having first and second sides.The first side can be a front side, and the second side a back side. Thewafer 30 can have substantially circular major surfaces. In certainimplementations, the wafer 30 can have a diameter of approximately 100mm, 150 mm, 200 mm, 300 mm, or greater. By way of an example, a 150 mm(also referred to as “6-inch”) GaAs substrate can have a thicknessranging from approximately 600 μm to 800 μm in some instances.

FIG. 2 depicts an enlarged view of a portion 31 of the wafer 30 ofFIG. 1. The wafer 30 can include a substrate layer 32 (e.g., a GaAssubstrate layer). The wafer 30 can further include a number of featuresformed on or in its front side. In the example shown, a transistor 33and a metal pad 35 are depicted as being formed the front side. Theexample transistor 33 is depicted as having an emitter 34 b, bases 34 a,34 c, and a collector 34 d. Although not shown in the figure, thecircuitry can also include formed inductors, capacitors, and source,gate, drain necessary for planar Field Effect Transistors. Suchstructures can be formed by processing on epitaxial layers that havebeen deposited on the substrate layer.

Overview of Evaporators

A thin film of metal can be deposited by evaporation onto one or morewafers as part of a process that may generally be referred to as“evaporation.” In some instances, this metal deposition can be part of afront side process for wafers that include at least a portion of atransistor device, for example, a heterojunction biopolar transistor(HBT) device. For instance, the transistor 33 illustrated in FIG. 2 maybe a HBT. In some instances, the end product of processing the GaAswafer can include at least one bipolar field effect transistor (BiFET)device. While the description herein may refer to a GaAs process, thefeatures described herein could apply to any type of semiconductorprocess that includes evaporation. For example, evaporation processesassociated with gain blocks and varactors, pseudomorphic high electronmobility transistors (pHEMT), light emitting diodes (LEDs), solar cells,or any other semiconductor structures on a semiconductor substrate couldimplement any combination of features described herein associated withevaporation. By way of example, a semiconductor substrate can include aGaAs substrate or a Si substrate.

A metal source in an evaporator can be heated by an energy source by anumber of methods, such as a thermal method, an electron-beam method, aflash evaporation method, and/or a resistive evaporation method.Although the following description may refer to a particular energysource, any combination of the features described herein may beimplemented with any energy source associated with evaporation known inthe art. The energy source can evaporate the metal source, therebycreating a metal vapor. A vacuum, which may be created by a vacuum pump,can allow the resulting vapor particles to travel directly to adeposition target, which may be on a GaAs wafer. The evaporated metalmay condense on the substrate to produce a thin film.

FIG. 3 illustrates an evaporator 100 suitable for evaporating metal ontoa plurality of substrates, such as wafers, according to one embodiment.Advantageously, the evaporator 100 may be used in processing any type ofsemiconductor wafer, for example, as described earlier. In oneparticular instance, the wafer includes HBT devices, which, in someinstances, can be BiFET devices. For example, the evaporator 100 may beused to evaporate metal over one or more epitaxial layers of a bipolartransistor. In some instances, the evaporator 100 may be used to deposita thin film of metal over at least a portion a collector, a base, anemitter and/or a gate area of a BiFET device. In the example provided inFIG. 2, the contacts may correspond to a top portion of the collector 34d, the base 34 a, 34 c and/or the emitter 34 b on the front side of thewafer 30. Alternatively or additionally, the evaporation may be used toform metal interconnects. The evaporator 100 may also be used forproviding contacts for the gate, source, and/or drain of a pHEMT, forproviding contacts for solar cells, and/or for other applications.

Titanium, platinum, gold, gold germanium alloy, nickel, copper,aluminum, tantalum, tungsten, palladium, iridium, antimony, beryllium,cadmium selenide, cadmium telluride, chromium, gallium antimonide,gallium arsenide, gallium nitride, gallium manganese arsenide, indium,indium phosphide, indium telluride, iron, magnesium, molybdenum,permalloy, scandium, silicon, silver, supermalloy, nickel vanadiumand/or other thin films may be deposited onto a wafer using theevaporator 100.

A metal vapor can be created by applying energy to a metal source 106 ina crucible 112. A shutter 114 can open so as to allow evaporation of atleast a portion of the metal source 106 into the evaporation chamber anddeposit a thin film on one or more wafers. The shutter 114 may openbased on the recipe on the tool. Different metal sources may be used inthe evaporator for different process steps. The shutter 114 may close inresponse to a first crystal monitor sensor 118 detecting one or morepredetermined conditions within the evaporation chamber. An example ofsuch a condition may include the first crystal monitor sensor 118detecting that a film of metal of a predetermined thickness has beendeposited on one or more wafers. However, the shutter 114 can block theevaporation stream to a first crystal monitor sensor 118 when in aclosed position. A second crystal monitor sensor 116 can monitorevaporation conditions when the shutter 114 is in the closed position.Software in a crystal monitoring system may be configured to prevent theshutter 114 from opening (e.g., by tool alarming) if there is a problemis detected. Some example problems include no metal source 106 or anundesired metal source 106 in the crucible 112. On the other hand, ifthe metal source 106 is in place and undesired conditions are notdetected, the shutter 114 may open based on the recipe. More detailregarding the monitoring crystal sensors 116, 118, the crucible 112, andthe shutter 114 will be provided later in connection with FIGS. 7 and 8.

An evaporation chamber of the evaporator 100 may include a wafer holder,such as a dome 102, configured to hold a plurality of wafers. While thefollowing description may reference a dome shaped wafer holder, otherwafer holders may be used in other embodiments. In some instances, thedome 102 may be comprised of stainless steel. And in someimplementations, the dome 102 may be configured to rotate. The dome 102may include a plurality of slots 104 configured to hold wafers. In someembodiments, the slots 104 may be configured to hold wafers having adiameter of about 6 inches or greater. Each slot 104 may include a holesized to expose a major surface of each wafer to vapor particles emittedfrom the metal source 106. Around the circumference of a hole, the slot104 may also include a lip or ridge configured to secure a wafer inplace.

With advances in technology, larger wafers may be used in semiconductormanufacturing processes, such as the processes described above inreference to FIGS. 1 and 2. For example, 6-inch wafers may be processedinstead of 4-inch wafers. The same manufacturing facility used toprocess previous wafers may be converted to process larger wafers, asmanufacturing facilities are very expensive to build and time consumingto set up for production. Part of the conversion to modify amanufacturing facility for production of larger wafers may includemodifying existing tools in the manufacturing facility, such as a waferholder in an evaporator, to accommodate a larger wafer size. Forexample, slots in a dome may be sized to hold fewer larger wafers. Bymodifying existing tools, substantial costs associated with newequipment and modification of the manufacturing facility may be avoided.

As the size of wafers increases, the number of wafers that can be heldin a wafer holder of a fixed size, such as the dome 102, decreases. Forexample, a dome with slots for 42 individual 4-inch wafers may only beable to hold 18 6-inch wafers because the corresponding surface area of19 6-inch wafers is greater than 42 4-inch wafers. Equipment limitationsmay make it difficult to increase the number of wafers held in a dome offixed size. The reduced number of slots to hold wafers may lead toundesirable results. For instance, evaporating metal onto fewer waferscan reduce evaporator capacity. This can impact the capacity of theentire production flow, particularly when the dome 102 can hold lessthan a multiple of the number of wafers that are processed in a previousprocessing step plus a test wafer.

The number of wafers held by a wafer holder can be related to processingsteps other than evaporation. Processing steps may process a group ofwafers in a wafer lot that includes a predetermined number of wafers. Insome instances, the wafer lot may include 20 wafers. While processing alot, it can be desirable to also process a test wafer, which may also bereferred to as a “witness wafer.” The test wafer can be used to measurea number of parameters, in order to monitor product quality of eachproduction run. For instance, the test wafer can be used to measureparameters related to thickness, sheet resistance, uniformity of themetal deposited, stress, and/or particles evaporated (e.g., to check fora clump of metal particles sometimes referred to as a “nodule”).Parameter values measured from the test wafer may be used to help keeptight control of particles and/or monitor any deviations from the normcan help ensure that products with reliability issues are not shipped tocustomers. Some reliability issues may relate to the reliability ofstandard capacitors and/or stacked capacitors in an end product.

Test Wafer Holder

Conventionally, a test wafer has been placed in the dome 102 of theevaporator 100 so that the test wafer can be exposed to substantiallythe same environment as production wafers. The environment inside theevaporator 100 is tightly controlled, and minor changes in position of awafer and/or components can affect the results of evaporation onto thewafer. As a result, it may be advantageous to position a test wafer sothat the test wafer will have characteristics, such as the parametersdescribed herein, that strongly correlate with production wafers. Insome instances, it may be desirable for the test wafer to besubstantially the same size as the production wafers so that the testwafer may see a number of the same effects related to wafer size as theproduction wafers. In other instances, it may be sufficient for the testwafer to be smaller than the production wafer because the measuredparameters may be adequate for determining reliability. For example, a4-inch test wafer may be used with 6-inch production wafers, in someimplementations.

If a sufficient number of slots 104 are not included in the dome 102 tofit at least one wafer lot and a test wafer, the evaporator capacity maydecrease. For example, if there are 20 slots 104 in the dome 102, theevaporator 100 may have a capacity reduced by 5% if 19 production wafersout of a wafer lot of 20 wafers, along with 1 test wafer, are positionedwithin the slots 104 of the dome 102 during evaporation. Preventing adecrease in evaporator capacity can have a significant impact on theproduction process. For example, preventing a 5% decrease in evaporatorcapacity may make purchasing an additional evaporator, which may costmore than $750,000, unnecessary. Moreover, it may be logisticallydifficult to find clean room space and appropriate facilities for anadditional evaporator. In addition, this clean room space may come atthe expense of another tool used in another processing step. As anotherexample, significant cost avoidance in precious metal costs may berealized by preventing a 5% decrease in evaporator capacity.

An alternative approach to positioning the test wafer within theevaporation chamber is illustrated in FIG. 3. A test wafer holder 108 isincluded in the evaporation chamber separate and spaced apart from thewafer holder, which is the dome 102 in the illustrated embodiment. Inthis way, an evaporator with a wafer holder that can hold only thenumber of wafers in a wafer lot can run at full capacity by positioningthe test wafer in the evaporation chamber separate from the waferholder. Advantageously, positioning the test wafer in a separate waferholder enables each slot 104 in the dome 102 to be used by productionwafers. More details regarding test wafer holders will be provided laterin connection with FIGS. 4, 5A and 5B.

As illustrated in FIG. 3, the test wafer holder 108 is coupled to across beam 110 at a location below a shaper 109. The shaper 109 and thecross beam 110 may be made of metal, such as stainless steel. The shaper109 may positioned within the evaporation chamber between the metalsource 106 and the dome 102, in order to improve the uniformity of metaldeposited on one or more of the wafers held in slots 104 of the dome102. The shaper 109 can block a portion of the metal vapor travelingbetween the metal source 106 and wafers in the dome 102, while the dome102 rotates during evaporation. This can result in thin films withgreater uniformity being deposited on at least one wafer positioned inthe dome 102 during evaporation.

Referring to FIG. 4, an evaporator 120 including a test wafer holder 122separate and spaced apart from a wafer holder 124 according to anotherembodiment is provided. Any combination of features of the evaporator120 may be implemented in connection with the evaporator 100. While onetest wafer holder 122 is shown in FIG. 4, more than one test waferholder 124 may be included in other embodiments.

As illustrated in FIG. 4, the test wafer holder 122 is attached to across beam 125 below a major surface of a shaper 126 between a metalsource 128 and the wafer holder 124. The test wafer holder 122 may beattached to the cross beam 125 below the major surface of the shaper 126that is closest in distance to the metal source 126. In otherembodiments (not illustrated), a second test wafer holder may beattached to the cross beam 125 on the opposite side of the evaporationchamber. The second test wafer holder may include any combination offeatures described in connection with the test wafer holder 122.Attaching the test wafer holder 122 to the cross beam 125 can provide arelatively sturdy, repeatable platform that may ensure that a test waferis consistently positioned in the test wafer holder 122 over time.

The test wafer holder 122 may be sized such that the test wafer holder122 does not interfere with the path of the vapor from the metal source128 to the wafer holder 124. At the same time, a test wafer positionedin the test wafer holder 122 may be in the direct path of the vapor fromthe metal source 128, which may make it more likely that a relativelygood correlation can be found between the characteristics of a testwafer and characteristics of a production wafer. These features of thetest wafer holder 122 may be implemented by sizing a major surface ofthe test wafer holder 122 such that it has a footprint that is nogreater than a footprint of the shaper 126 in a direction substantiallyparallel to a bottom and/or a top of the evaporation chamber, in someembodiments. In other embodiments, the footprint of the test waferholder 122 may be larger than the footprint of the shaper in the pathmetal vapor travels from the metal source 128 to wafers positioned inthe wafer holder 124.

In addition, whether or not the test wafer holder 122 includes a testwafer may not impact the uniformity of thin film metal deposition ontowafers held in slots of the wafer holder 124, since the shaper 126 willblock metal vapor if the test wafer holder 122 does not include a testwafer.

A wafer positioned in the test wafer holder 122 and another waferpositioned in the wafer holder 124, which can be dome shaped, may bedifferent distances from the metal source 128. For instance, a distancefrom a test wafer in the test wafer holder 122 to the metal source 128may be approximately 750 mm to 1500 mm and a distance from a wafer inthe wafer holder 124 to the metal source 128 may be approximately 625 mmto 1250 mm.

The different distances from the metal source may result in differencesin thin films of metal deposited on these wafers, which may in turnresult in different values for parameters that may be used to monitorthe quality of wafers in a production run, for example, as describedabove. In some instances, the differences in parameter values for awafer positioned in the test wafer holder 122 during evaporation andanother wafer positioned in the wafer holder 124 during evaporation maybe scaled to account for the difference in distance from the metalsource 128. For example, the sheet resistance can be scaled by ageometric factor that takes into account the difference in distancebetween the test wafer holder 122 and the wafer holder 124 using similartriangles.

FIGS. 5A and 5B provide an example design of a test wafer holder 130.Any combination of features of the test wafer holder 130 may beimplemented in connection with the test wafer holders 108, 122 describedabove. The test wafer holder 130 may be positioned to enable a testwafer to capture a metal deposition that correlates with a metaldeposition on a production wafer positioned in a wafer holder, such as adome, without impeding deposition of the production wafer positioned inthe wafer holder as also described above. The test wafer holder 130 maybe made of stainless steel, aluminum, copper, chromium, or othersuitable metals.

FIG. 5A shows a side view of the test wafer holder 130. The test waferholder 130 may have a height h1, which may approximate the distance of atest wafer from a shaper. A top surface 132 of the test wafer holder 130may be separated from the shaper by the thickness of a cross beam, whichmay be approximately 20 mm. The height h1 of the test wafer holder maybe selected to be large enough such that a wafer may easily bepositioned in the test wafer holder 130, while being small enough toprevent the test wafer holder 130 from substantially interfering withdeposition onto production wafers positioned in a separate wafer holder,such as a dome. Yet some interference may be tolerated in certainapplications. In some implementations, the height h1 of the test waferholder may be approximately 20 mm to 40 mm, for example. The height h1can be selected to allow sufficient room for manual loading andunloading of the test wafer with a wafer vacuum wand or other implement.

The test wafer holder 130 may include the top surface 132 that may beattached to the cross beam within an evaporation chamber through acenter hole ranging from approximately 4 mm to 8 mm in diameter. Athickness h2 of the top surface 132 of the test wafer holder 130 may beapproximately 2 mm to 8 mm. As illustrated in FIG. 5A, the test waferholder 130 may include a plurality of posts 134 that can connect the topsurface 132 of the wafer holder 130 to a bottom portion of the testwafer holder 130. In some implementations, a thickness h3 of the bottommay be approximately 3 mm to 8 mm. The posts 134 may be positioned toallow a wafer to be easily placed in a slot 136 that includes a hole 138in a bottom portion 140 of the test wafer holder 130. The test waferholder 130 may have four posts 134 in certain instances. However, anysuitable number of posts or other supporting structures can be employed.

FIG. 5B illustrates a top view of the bottom portion 140 of the testwafer holder 130. The bottom portion 140 may include a slot that mayhave, for example, a hole 138 and a lip or ridge 142 for supporting atest wafer. The test wafer may be placed in the test wafer holder 130such that a major surface of the test wafer may be in a direct path of avapor generated by a metal source in an evaporation chamber. In someimplementations, the hole 136 may have a width d1 that is smaller thanthe diameter of a test wafer by approximately 2% to 6% at the widestpoint and a width d2 from one end of the lip 142 to the other end of thelip 142 may be approximately the same size as the wafer diameter or afew percent larger across the widest point of the hole 136. In suchimplementations, the width d3 of the bottom portion 140 of the testwafer holder 130 may be approximately 10 mm to 30 mm larger than thewafer diameter at the widest point. The lip 142 may be sized toaccommodate a portion of the test wafer that may not be intended to beexposed to the vapor.

As described above, it may be desirable for a footprint of the testwafer holder 130 to be no greater than the footprint of a shaper in thepath that vapor travels from a metal source to a production wafer in awafer holder. Due to the shape of certain shapers, opposing ends 144,146 of the top 132 and the bottom portion 140 of the test wafer holder130 may be substantially flattened as illustrated in FIGS. 5A and 5B,instead of continuing a substantially circular shape. For example, atest wafer holder can have a footprint contained within a footprint of ashaper in a direction substantially parallel to a bottom and/or a top ofan evaporation chamber. Alternatively or additionally, a test waferholder may have a footprint contained within a footprint of a shaperalong a path that the metal source travels to production wafers. Thismay allow the test wafer holder to avoid interfering with depositiononto the production wafers.

FIG. 6 provides a flowchart of a method 150 of processing one or moresemiconductor wafers, such as GaAs wafers. The method 150 may use anevaporator that includes any combination of features of the evaporatorsdescribed herein, such as the evaporators 100, 120. Any combination ofthe features of method 150 may be embodied in a non-transitory computerreadable medium and stored in non-volatile computer memory. Whenexecuted, the non-transitory computer readable media may cause some orall of the method 150 to be performed. It will be understood that themethods discussed herein may include greater or fewer operations and theoperations may be performed in any order, as necessary.

The method 150 may begin at block 152 in which an evaporation chamber isprovided. The evaporation chamber may include a dome configured to holda plurality of wafers. In some implementations, the dome is configuredto hold wafers having a diameter of at least about 6 inches. Thesewafers may be held in slots of the dome. The dome may be configured torotate about an axis substantially perpendicular to major surfaces of ashaper in the evaporation chamber of the evaporator. Alternatively oradditionally, dome may be configured to rotate about an axis of a linefrom a metal source to the center of the dome. The evaporation chambermay also include a test wafer holder separate and/or spaced apart fromthe dome. In certain implementations, the test wafer holder may beattached to a cross beam below a shaper. In some implementations, theshaper may have a footprint at least as large as a footprint of a testwafer holder in the path metal vapor travels between the metal sourceand the semiconductor wafer positioned in the dome. This may allow thetest wafer to be in a direct path of metal vapor from the metal sourceduring evaporation, without interfering with evaporation onto productionwafers positioned in slots of the dome. In other implementations, thefootprint of the test wafer holder may be larger than the shaper in thepath metal vapor travels between the metal source and the semiconductorwafer positioned in the dome.

Next, one or more semiconductor wafers may be positioned within theevaporation chamber. One or more semiconductor production wafers mayalso be positioned within the dome at block 154. The one or moresemiconductor production wafer may be GaAs wafers. A semiconductor testwafer may be positioned in the test wafer holder at block 156. Thepositioning actions may be carried out in any sequence orsimultaneously. Each of these wafers may have a major surface exposed toa path of vapor traveling from the metal source. This may beaccomplished via holes in slots in the dome and/or holes in the testwafer holder. These holes may be sized to expose substantially all ofthe wafers to metal vapor. An outer ring of the wafers may be blockedfrom the metal vapor by a lip or ridge around the holes.

Energy, such as heat, may be applied to a metal source at block 158.When energy is applied to the metal source, metal from the metal sourcemay be evaporated into the evaporation chamber. The metal source mayinclude, but is not limited to, titanium, platinum, gold, nickel, copperand/or gold germanium alloy.

Metal may be evaporated onto a semiconductor wafer positioned in thetest wafer holder, while evaporating metal onto the semiconductor wafers(e.g., GaAs wafers) positioned in the dome at block 160. This maydeposit a thin film of metal over each of the wafers. At least a portionof the thin film of metal may be removed by subsequent processing steps.In a number of implementations, at least a portion of the metalevaporated onto the semiconductor wafer may function as a contact on aHBT device for a collector, a base, and/or an emitter. According to someof these implementations, the device may combine an HBT and FET tocreate a BiFET device. In other implementations, evaporation may be partof a process of forming functional varactors or gain blocks, pHEMTs,LEDs, solar cells, or other structures.

After metal deposition, one or more parameters of the test wafer may bedetermined at block 162. Such parameters may be associated with anymeasurement that may be indicative of quality and/or reliability ofstructures formed on the wafer. Example parameters may be associatedwith, among other things, thickness, sheet resistance, uniformity of themetal deposited, stress, and/or particles evaporated. At block 164, aquality and/or reliability assessment of the production semiconductorwafers can be made based at least in part on the one or more parametersdetermined at block 162. Production wafers that are determined to have aquality and/or reliability that satisfies a predetermined threshold maycontinue to be processed, and production wafers that are determined tobe of a quality that does not satisfy the predetermined threshold maynot be shipped to customers. The predetermined threshold may be based atleast in part on any combination of the parameters of the test wafersdescribed herein. The predetermined threshold may also be adjusted overtime and/or for different end products. For example, the predeterminedthreshold may depend on a particular application of end products thatinclude at least a portion of a production semiconductor wafer.

Detecting Evaporation Conditions

In addition to using test wafers to assess metal deposition onproduction wafers, evaporators may also be configured to monitor theprogress of evaporation and/or metal deposition, while metal isevaporated in the tightly controlled environment of an evaporationchamber. Some evaporators use crystal monitoring systems to monitormetal deposition. Although the following description provides examplesdescribed in connection with crystal monitoring systems, the conceptsdescribed may be applied to any other suitable systems and/or methodsusing other hardware that can monitor conditions of metal deposition.While features related to crystal monitors are described below inconnection with an evaporator, the concepts discussed may be applied toa variety of vacuum deposition systems, including without limitationelectron beam, sputtering, molecular beam epitaxy (MBE), chemical vapordeposition (CVD) including plasma enhanced CVD, metal organic CVD, photovoltaic CVD, organic light emitting diode (OLED), and/or atomic layerdeposition (ALD) processes.

Crystal monitor systems may be included to monitor deposition ratesand/or accumulated metal thickness on a wafer. A crystal monitor systemmay include a crystal monitor sensor coupled to an oscillator and anexternal monitoring system configured to control the oscillator andobtain measurements associated with the crystal sensor. A crystaloscillates at a resonant frequency that may depend on a thickness and/ora mass of the film deposited onto the crystal. By applying a voltage,for example, via an oscillator, across faces of a suitably shapedpiezoelectric crystal, the crystal may be distorted and/or change shapein proportion to the applied voltage. At certain discrete frequencies ofapplied voltage, a condition of sharp and/or repeatableelectro-mechanical resonance may be seen. In some instances, the crystalmonitor sensor may include a plurality of inputs. These inputs may beassigned to different materials, averaged for accurate depositioncontrol, and/or configured for a dual sensor. Certain crystal monitorsystems may be capable of measuring thickness of less than a singleatomic layer with 0.5% accuracy. By measuring a resonant frequency of anoscillating crystal, a thickness of a metal and/or a dielectric coatingmay be determined within 1 Angstrom, in some instances.

Conventional evaporators may position a crystal monitor sensor near oneor more substrates onto which a thin film is deposited. In this way, thecrystal monitor sensor may be able to take accurate measurements thatclosely correspond to material being deposited on the one or moresubstrates because the crystal monitor sensor may be exposed to asubstantially similar environment as a substrate. In response to thecrystal monitor system detecting that a desired thickness of materialhas been deposited on the one or more substrates, a shutter can close soas to block additional vapor from the metal source from being depositedonto the one or more substrates.

Some evaporators may be used to deposit a number of different metallayers onto a substrate. For example, certain evaporators may be used todeposit a thin film of metal over several different features of asubstrate during separate processing steps. Various metals may beselected based on how they interact with the materials over which theyare formed. In some instances, different metals may be deposited over acollector, a base, and/or an emitter of a bipolar transistor, such as aHBT device, so that the metal may function as a contact. Differentmetals may also be used in connection with any of the devices describedherein, such as BiFETs, pHEMTs, and others. A single evaporator may beused to perform each of these metal depositions. Alternatively oradditionally, different metals may also be deposited on the substrateusing the single evaporator to form features, including withoutlimitation a source, a gate, and/or a drain of a field-effect transistorand/or interconnects between circuit elements.

When using an evaporator for depositing a plurality of different metalsources onto a substrate, a particular metal source should be placedwithin the evaporator corresponding to a desired metal to be evaporatedonto the substrate. However, in some cases, an incorrect metal is placedwithin the evaporator. For example, a metal for a base contact may beloaded in the evaporator as a metal source for processing related toforming an emitter contact, instead of a different metal for the emittercontact. If the incorrect metal is deposited on a substrate, thesubstrate can be damaged, and even ruined. With a crystal monitor sensorpositioned near a substrate on which the metal is evaporated, as in someconventional evaporators, the crystal monitor system may not be able todetect which metal source is being evaporated until at least some metalis deposited onto the substrate. At that point, the substrate mayalready be damaged.

Alternatively or additionally, other conditions related to evaporationmay indicate that the evaporation chamber is not working as desiredand/or that the metal source should not be deposited onto the wafer. Forexample, a crystal monitoring system may be used to detect that themetal source includes impurities above a predetermined threshold level.As another example, a melt of the wrong material could be in the pocket,or the wrong material could be added to the correct melt. In addition,the melt may be left out of the crucible pocket entirely. In anelectron-beam evaporator, the crystal monitor can also detect a beamintensity strength and/or a beam position in the pocket (e.g., if thebeam is properly centered or not). Crystal monitor sensors positionednear substrates may not be able to detect these undesirable conditions,until at least some metal is deposited onto the substrate.

FIGS. 7A and 7B illustrate an evaporator 170 with two crystal monitorsensors according to an embodiment. One of the two crystal monitorsensors may be used to monitor evaporation associated with a metalsource before metal from the metal source is deposited onto a substrate,by preventing a shutter from opening under certain conditions. This maybe accomplished, for example, by selectively allowing deposition ontothe substrate only if certain conditions are detected.

The evaporator 170 can be used, for example, in connection with theprocess described above in connection with FIG. 1. Any combination offeatures of the evaporator 170 may be implemented in connection with theevaporators described herein, for example, the evaporators 100, 120.Like the evaporator 120, the evaporator 170 includes a wafer holder 124shaped as a dome, a metal source 128, and a shaper 126 positionedbetween a metal source 128 and the wafer holder 124. As shown in FIGS.7A and 7B, the evaporator 170 can also include a shutter 172, a firstcrystal monitor sensor 174, a second crystal monitor sensor 176, and acrucible 178.

In FIG. 7A, the shutter 172 is shown in a closed position. The shutter172 can block vapor from the metal source 128 from being deposited ontoa wafer positioned in a slot of the wafer holder 124 when in the closedposition. In contrast, FIG. 7B illustrates the shutter 172 in an openposition that allows metal from the metal source to be deposited on awafer positioned within the wafer holder 124. The shutter 172 may bemade of any suitable material that can block metal vapor from the metalsource. The shutter 172 may also be any suitable shape that can blockmetal vapor from traveling from the metal source to one or moresubstrates on the wafer holder 124.

The shutter 172 may close in response to the first crystal monitorsensor 174 detecting a condition indicating that a suitable thin filmhas been deposited on one or more substrates positioned within slots ofthe wafer holder 124. For instance, the first crystal monitor sensor 174may detect that a thickness of metal deposited onto one or moresubstrates positioned in the wafer holder 124 have reached a desiredthickness, and then the shutter 172 may close. The first crystal monitorsensor 174 may be positioned within the evaporation chamber such thatthe first crystal monitor sensor 174 observes substantially similarconditions to the one or more substrates positioned in slots of thewafer holder 124. For example, the first crystal monitor sensor 174 maybe positioned in the same plane as the wafer holder 124. For example, ahole may be made in the top of the wafer holder 124 to allow the crystalmonitor sensor 174 to be exposed to the metal vapor.

The second crystal monitor sensor 176 may also be included in theevaporation chamber to monitor evaporation conditions before the shutter172 opens so as to prevent deposition onto one or more substratespositioned in slots of the wafer holder 124 unless certain conditionsare met. For instance, as the metal source 128 in the crucible 178 isbeing heated to ramp up evaporation, the second crystal monitor sensor176 may detect whether or not the metal source 128 corresponds to adesired metal. This can verify that the correct metal source has beenadded to the evaporator 170 for a particular processing step. In someinstances, the second crystal monitor sensor 176 may receive data atdifferent inputs corresponding to a particular metal source. Forexample, a first input may correspond to detecting a first metal sourceand a second input may correspond to a second metal source.Alternatively or additionally, the second crystal monitor sensor 176 maydetect a faulty metal source 128 that should not be evaporated onto awafer. In some cases, the faulty metal source may contain impurities orother defects. Other examples of conditions that can be detected by thesecond crystal monitor 176 include detecting an incorrect melt within apocket of the crucible 178, detecting an undesired material added to acorrect melt, detecting a position of the melt within the cruciblepocket (e.g., the melt may be left out of the crucible pocket entirely).The second crystal monitor 176 can also detect a beam intensity strengthand/or if the beam is properly centered or not in the crucible pocket inelectron beam evaporators.

The shutter 172 may open only if the second crystal monitor sensor 176detects a desired condition for evaporation, such as that the correctmetal source is being evaporated. This may prevent damage to the one ormore wafers on which metal from the metal source is being deposited. Inorder to perform such monitoring, the second crystal monitor sensor 176may be positioned such that the second crystal monitor sensor 176 maymonitor evaporation conditions when the shutter 172 is closed.

FIG. 8A illustrates a shutter system 180 according to one embodiment. Acrucible 182 may hold a metal source 184 to be evaporated in anevaporation chamber. Energy, such as heat, may be applied to the metalsource 184 in the crucible 182 in order to heat the metal source 184.The crucible 182 may be made of any suitable material that can withstandthe heat directed onto the metal source 184 without sustainingsubstantial damage. In some embodiments, the crucible 182 may be made ofcopper. A crystal monitor 186 may be positioned such that a crystalmonitor sensor 186 may detect desired conditions for metal evaporationwhen a shutter 188 is in a closed position. In some implementations, theshutter 188 may create a sealed space for initial evaporation byblocking the opening of the crucible 182 and/or an aperture throughwhich metal vapor travels from the metal source 184 in the crucible 182.This may prevent metal vapor from reaching a substrate in theevaporation chamber onto which metal from the metal source 184 isevaporated.

FIG. 8B illustrates an example crystal monitor sensor 190. Any of thecrystal monitor sensors referenced herein may include any combination offeatures of the crystal monitor sensor 190. The crystal monitor sensor190 may include a relatively thin crystal 192 positioned between twoelectrodes 194, 196. As illustrated in FIG. 8B, the crystal 192 maysubstantially have a shape of a disc with the two electrodes 194, 196physically and electrically contacting two opposite major surfaces ofthe crystal 192. In some instances the crystal 192 may be quartz. Inother instances, the crystal 192 may include without limitation galliumphosphate (GaPO₄), langasite, and/or langatite. The electrodes 194, 196may be made of gold, silver, aluminum, and/or other electricallyconductive materials. Current and/or voltage signals may be applied tothe electrodes 194, 196 in order to monitor conditions related todeposition. For example, such signals may be generated by an oscillator.

FIG. 9 is a method flowchart for a process 200 for monitoringevaporation conditions according to one embodiment. The process 200 maybe implemented using an evaporator that includes any combination offeatures of the evaporators described herein, such as the evaporators100, 170. Any combination of the features of the process 200 may beembodied in a non-transitory computer readable medium and stored innon-volatile computer memory. When executed, the non-transitory computerreadable media may cause some or all of the process 200 to be preformed.The process 200 may begin by initiating heating of a metal source atblock 202. Energy can be applied to the metal source via any of themethods described herein. This may cause at least a portion of the metalsource to evaporate.

While evaporation is ramping up, a sensor may detect for undesiredconditions at block 204. The sensor may be any of the crystal monitorsensors referenced herein. The undesired conditions may include anycondition that indicates that the evaporator is not properly configuredto deposit a thin film onto one or more substrates. The undesiredconditions may be associated with any of the conditions mentionedherein. For instance, at least one of the undesired conditions may beassociated with a deposition rate and/or a property of the metal source,such as a composition characteristic or impurity level of the source.

Next, at block 206, a shutter may be opened. This can allow metal fromthe metal source to travel through the evaporation chamber to one ormore substrates positioned on a substrate holder. Until the shutter isopened, metal vapor may be blocked from reaching the one or moresubstrates on the substrate holder. Thus, metal deposition on the one ormore substrates may begin when the shutter is opened. In some instances,the shutter may be opened only if an undesired condition is notdetected.

Metal may be evaporated onto the one or more substrates at block 208.This may deposit a thin film of metal onto the one or more substrates.The thin film may cover any of the features of the one or moresubstrates described herein. At least a portion of the deposited metalmay function as, for example, a contact, a metal interconnect, and/or agate. The one or more substrates may include one or more GaAs wafers orany other substrates mentioned herein. The substrates may have adiameter of at least 6 inches.

CONCLUSION

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense, as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” The words “coupled” or connected“, asgenerally used herein, refer to two or more elements that may be eitherdirectly connected, or connected by way of one or more intermediateelements. Additionally, the words “herein,” “above,” “below,” and wordsof similar import, when used in this application, shall refer to thisapplication as a whole and not to any particular portions of thisapplication. Where the context permits, words in the above DetailedDescription using the singular or plural number may also include theplural or singular number respectively. The word “or” in reference to alist of two or more items, that word covers all of the followinginterpretations of the word: any of the items in the list, all of theitems in the list, and any combination of the items in the list.

Moreover, conditional language used herein, such as, among others,“can,” “could,” “might,” “may,” “e.g.,” “for example,” “such as” and thelike, unless specifically stated otherwise, or otherwise understoodwithin the context as used, is generally intended to convey that certainembodiments include, while other embodiments do not include, certainfeatures, elements and/or states. Thus, such conditional language is notgenerally intended to imply that features, elements and/or states are inany way required for one or more embodiments or that one or moreembodiments necessarily include logic for deciding, with or withoutauthor input or prompting, whether these features, elements and/orstates are included or are to be performed in any particular embodiment.

The above detailed description of certain embodiments is not intended tobe exhaustive or to limit the invention to the precise form disclosedabove. While specific embodiments of, and examples for, the inventionare described above for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. For example, while processesor blocks are presented in a given order, alternative embodiments mayperform routines having steps, or employ systems having blocks, in adifferent order, and some processes or blocks may be deleted, moved,added, subdivided, combined, and/or modified. Each of these processes orblocks may be implemented in a variety of different ways. Also, whileprocesses or blocks are at times shown as being performed in series,these processes or blocks may instead be performed in parallel, or maybe performed at different times.

The teachings of the invention provided herein can be applied to othersystems, not necessarily the systems described above. The elements andacts of the various embodiments described above can be combined toprovide further embodiments.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the disclosure. Indeed, the novel methodsand systems described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and systems described herein may be made withoutdeparting from the spirit of the disclosure. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the disclosure.

1. A method of processing a semiconductor wafer, the method comprising:providing an evaporator including a dome configured to hold a pluralityof wafers including the semiconductor wafer, and a test wafer holderseparate from the dome; heating a metal source so as to evaporate metalfrom the metal source within a chamber of the evaporator; andevaporating metal onto a semiconductor test wafer positioned in the testwafer holder, while evaporating metal onto the semiconductor waferpositioned in the dome.
 2. The method of claim 1 wherein thesemiconductor wafer is a GaAs wafer.
 3. The method of claim 2 wherein atleast a portion of the metal evaporated onto the GaAs wafer isconfigured to function as a contact on a HBT device for at least one ofa collector, a base, and an emitter.
 4. The method of claim 1 whereinthe test wafer is an Si wafer.
 5. The method of claim 1 wherein adiameter of the semiconductor wafer is at least about 150 mm.
 6. Themethod of claim 1 wherein the test wafer holder is attached to a crossbeam supporting a shaper, and wherein the shaper has a footprint aboutat least as large as a footprint of test wafer holder in the path metalvapor travels between the metal source and the semiconductor waferpositioned in the dome.
 7. The method of claim 1 further comprising:detecting a condition related to metal evaporation using a crystalmonitor; and keeping a shutter in a closed position in response todetecting the condition, so as to prevent metal deposition of the metalsource onto the GaAs wafer and the test wafer in the chamber.
 8. Themethod of claim 1 further comprising rotating the dome while evaporatingmetal onto the semiconductor wafer.
 9. The method of claim 1 wherein thesemiconductor test wafer is held in the test wafer holder by a ridge,the ridge surrounding a hole in the test wafer holder.
 10. A method ofprocessing a GaAs wafer, the method comprising: providing an evaporatorincluding a wafer holder having slots configured to hold a plurality ofwafers having a diameter of at least about 150 mm, and a test waferholder configured to hold at least one test wafer, the test wafer holderspaced apart from the wafer holder; positioning the GaAs wafer in a slotof the wafer holder; applying an energy source to a metal source so asto evaporate metal from the metal source within the evaporator; anddepositing a thin film of metal from the metal source onto the GaAswafer, the GaAs including at least a portion of at least one of a BiFETdevice, a varactor, a gain block, and a pseudomorphic high electronmobility transistor (pHEMT) device.
 11. The method of claim 10 whereinthe wafer holder is configured to hold a wafer having a thickness of nogreater than about 800 μm.
 12. The method of claim 10 wherein the metalsource comprises at least one of gold, gold germanium alloy, platinum,titanium, nickel, and copper.
 13. A method comprising: positioning afirst semiconductor wafer in a first wafer holder of an evaporator;positioning a second semiconductor wafer in a second wafer holder of theevaporator, the second wafer holder separate and spaced apart from thefirst wafer holder; and evaporating metal onto the first semiconductorwafer positioned in the first wafer holder while the secondsemiconductor wafer is positioned in the second wafer holder.
 14. Themethod of claim 13 wherein evaporating metal includes depositing metalonto at least two GaAs wafers.
 15. The method of claim 13 wherein thesecond wafer holder is coupled to a cross beam supporting a shaper. 16.The method of claim 15 wherein positioning the second semiconductorwafer in the second wafer holder includes positioning the secondsemiconductor wafer between a metal source being evaporated and theshaper.
 17. The method of claim 15 wherein the second wafer holderincludes a bottom portion having a hole surrounded by a ridge, the ridgeconfigured to hold the second semiconductor wafer in place.
 18. Themethod of claim 13 wherein the first wafer holder is configured to holda wafer having a diameter of at least about 150 mm.
 19. The method ofclaim 13 wherein the first wafer holder is configured to hold a waferhaving a thickness of no greater than about 800 μm.
 20. The method ofclaim 13 wherein the second wafer holder is configured to hold a singlesemiconductor wafer.